testing alternatives for salt wedge management in … · the greek territory through the straits of...

INTRODUCTIONEstuaries form the transition zone between the

inland freshwater and seawater. Due to their posi-

tion they are characterized by some unique fea-

tures and properties, one of which is the intrusion

of seawater upstream of the river mouth (when-

ever river topography is below mean sea level),

mainly caused by its higher density. Thus, one of

the problems of water management in estuarine

areas is the control of salinity, as salinity limits

have been assessed for the use of water for vari-

ous purposes, including irrigation.

Several methods have been proposed to control

or prevent salt intrusion. These include, among

Global Nest: the Int. J. Vol 5, No 2, pp 107-118, 2003

Copyright© 2003 GLOBAL NEST

Printed in Greece. All rights reserved

TESTING ALTERNATIVES FOR SALT WEDGE MANAGEMENT

IN AN ESTUARY WITH THE USE OF MONITORING

AND A MATHEMATICAL MODEL

1 Democritus University of Thrace, School of Engineering

Department of Environmental Engineering

Laboratory of Ecological Engineering and Technology

67100 Xanthi, Greece2 National Agricultural Research Foundation, Fisheries Research Institute

64007 Nea Peramos, Kavala, Greece

*to whom all correspondence should be addressed:

fax: +30-25410 78113

e-mail: [email protected]

ABSTRACTThe intrusion of salt wedge in rivers is a natural phenomenon, which occurs in many estuaries. Saline

water tends to propagate upstream from the river mouth, due to the limited freshwater and the tidal

and density currents developed, resulting in deterioration of water quality in the lower river reach.

Several methods to control the salt wedge have been employed, including the construction of inflatable

dams or gates. A promising method of control is the use of an air curtain. In this study, a two-dimen-

sional, laterally averaged numerical model has been developed to describe salt wedge intrusion. This

model provided necessary hydraulic parameters, which were used in air curtain design theory to evalu-

ate the application of the air curtain method in a particular estuary system. The application takes place

in the estuary of Strymon River in Northern Greece, where the limited discharge of freshwater, main-

ly caused by the construction of Kerkini dam, results in the creation and upstream intrusion of a salt

wedge in the summertime, affecting water quality and making water unsuitable for irrigation uses.

KEYWORDS: saline intrusion, salt wedge, estuary management, air curtain, numerical modeling,

Strymon Estuary.

K.I. HARALAMBIDOU1

G.K. SYLAIOS1,2

V.A. TSIHRINTZIS1,*

Selected from papers presented at the 8th Conference

on Environmental Science and Technology,

8 - 10 September 2003, Lemnos, Greece.

14-Haralambidou.qxd 9/11/2005 10:04 07

others the construction of small movable dams at

the river mouth and the increase of the roughness

of the river bottom. One proposed method is the

use of an air curtain, which may reduce or prevent

totally the intrusion of the wedge. According to

this method, compressed air is pumped perpen-

dicularly to the flow from a perforated pipe,

placed on the river bed across the channel, form-

ing a vertical air curtain which acts as a wall to the

intrusion. The use of the air curtain has been

studied through limited laboratory experiments

and has been applied in several navigation locks

(e.g., Abraham and Burgh, 1964; Raman and

Arbuckle, 1989, Hamilton et al., 2001).

This paper presents an application of the air cur-

tain technology in the estuary of Strymon River,

Northern Greece, where the limited discharge of

freshwater, mainly caused by the upstream con-

struction of Kerkini dam, results in a salt wedge

intrusion, affecting water quality and making

water unsuitable for irrigation uses. To deal with

the problem of the presence of salt wedge in

Strymon, an extended study has been undertaken,

which includes three major parts: (1) design and

application of an extensive field data collection

program along the river estuary (Haralambidou

et al., 2003a); (2) development of a numerical

model, which is used to test alternatives to control

the salt wedge intrusion (Haralambidou et al.,

2003b); and (3) testing of the effectiveness of the

air curtain method with the use of the model

(Haralambidou et al., 2003c). In this paper, the

numerical model and its use in testing the applic-

ability of the air curtain method in Strymon River

Estuary are presented.

MATERIALS AND METHODSNumerical model descriptionA two-dimensional, laterally-intregrated, explicit,

finite-difference numerical model has been devel-

oped to describe the characteristics of the salt

wedge intrusion in the estuary of Strymon River.

The governing equations of the model are based

on the principles of conservation of volume,

momentum and mass.

Considering hydrostatic pressure, the Boussinesq

approximation, and a Cartesian coordinate sys-

tem with the origin at the sea surface of the estu-

ary mouth (mean sea level), in which the x-axis is

directed upstream and the z-axis is directed

upward, the flow governing equations read:

The laterally integrated continuity equation:

(1)

The vertically-integrated continuity equation:

(2)

The momentum conservation equation:

(3)

where:

(4)

The salt balance equation:

(5)

The equation of state:

(6)

where: x and z = horizontal and vertical spatial

coordinates, cm; t = time, s; ρ(x,z,t) = water den-

sity, g cm-3; u(x, z, t) and w(x, z, t) = horizontal

and vertical laterally-averaged velocities, cm/s;

B(x, z) = channel width, cm; Bç = width at the

free surface, cm; g = acceleration due to gravity,

cm s-2; η(x, t) = sea surface displacement, cm; d =

water depth below mean sea level, cm; NX and NZ

= horizontal and vertical eddy viscosity coeffi-

cients, cm2 s-1; KX and KZ = horizontal and verti-

cal eddy diffusivity coefficients, cm2 s-1; k =

empirically determined drag coefficient, which is

expressed as: , where n the Man-

ning roughness coefficient, s cm-1/3; S(x, z, t) =

108 HARALAMBIDOU et al.

14-Haralambidou.qxd 9/11/2005 10:04 Page 108

salinity, psu; ρo = 0.9995 g cm-3 (density of fresh-

water); and á, â = constants, functions of tem-

perature. For water temperature of 15oC, á =

1.00059 and â = 7.57x10-4.

A first order turbulence closure scheme was used

for the parameterization of KZ and NZ eddy coef-

ficients, considering their dependence on the

local Richardson number Ri. In the present

model, the formulations of KZ and NZ are

(Blumberg, 1975, 1977):

(7)

for (8)

for (9)

where: k1 = 0.5 cm2 s-1, ãc = Kc/Nc for the critical

condition of stratification (Ric = 10, ãc ∼0.05-1.2)

(Yamada, 1975).

Instantaneous values of KZ and NZ are strongly

dependent on flow velocity, Manning's roughness

coefficient and vertical stratification (Sylaios,

1994; Parissis et al., 2001).

To eliminate non-linear instabilities in the model

results, an artificial horizontal viscosity term was

introduced (Smagorinsky, 1963). Horizontal eddy

viscosity and diffusivity coefficients KX and NX

were computed by the model, based upon the for-

mulation of:

where: c an adjustable constant and Äx the hori-

zontal grid spacing. Therefore, from this artificial

viscosity term, wherever the horizontal gradients

of velocity are large, the eddy viscosity will

become large.

The air curtain methodSeveral methods have been proposed to control

or prevent intrusion. One of them is the imple-

mentation of an air curtain (Abraham and Burgh,

1964; Tuin et al., 1991; Kerstma et al., 1994;

Hamilton et al., 2001). Nakai and Arita (2002)

presented experimental results on control of salt

wedge intrusion in rivers using this method. Their

theory is used in this study, based on the following

short description.

The salt wedge is influenced by an air curtain in

various ways. Nakai and Arita (2002) classified

the salt wedge behavior in three types, as illus-

trated in Figure 1. Under Type I, salt water

intrudes into the upstream side over the air cur-

tain along the channel bottom. This type appears

when the kinetic energy provided by the buoyan-

cy due to the air curtain is small. Under Type II,

saltwater is controlled downstream of the curtain

by the strong upward flow. Under Type III, salt-

water is also controlled downstream of the air

curtain, but some portion enters the upstream

side along the channel bottom. Obviously the pre-

ferred condition is Type II.

Nakai and Arita (2002) found that external forces

dominating the operation of the system are the

following three: the buoyancy due to the air cur-

tain, A, the intrusion force of the salt wedge, B,

and the inertial force of the freshwater flow, R.

All parameters can be written in velocity dimen-

sions (i.e., m/s), as follows (Nakai and Arita,

2002):

A = (qa g)1/3 ;

B = (g′ ha)1/2 ; (10)

R = g f /d

where: qa = discharge per unit width of air; g = grav-

itational acceleration; g ′ = reduced gravitational

acceleration [ ]; ñs and ñf are the

densities of saltwater and freshwater, respective-

ly; ha is the salt wedge thickness at the position of

the air curtain maker, in the absence of the air

curtain; qf = discharge per unit width of freshwa-

ter flow; and d = total water depth. The behavior

of a salt wedge around an air curtain depends on

both ratios A/B and A/R, as shown in Figure 2.

Description of the study areaSalt wedge intrusion is observed at the estuary of

Strymon River in Northern Greece. Strymon is a

transboundary river, with a total channel length of

315 km and a total drainage area of 18,329 km2,

39.8% of which belongs to Greek territory

(Hatzigiannakis, 2000). With a NW-SE direction,

109TESTING ALTERNATIVES FOR SALT WEDGE MANAGEMENT IN AN ESTUARY


Strymon River enters the Serres drainage area in

the Greek territory through the straits of Roupel,

passes the Kerkini dam located about 70 km

upstream of the estuary and flows into the

Strymonikos gulf in the North Aegean Sea

(Figure 3).

The present study involves the estuary system of

the river and particularly the area upstream of the

river mouth up to the straits of Amphipolis, a dis-

tance of approximately 6-8 km. The Strymon

Estuary is of the coastal plain type, forced at its

mouth by a micro-tidal M2-sinusoidal wave, with a

spring range of approximately 0.45 m and a neap

range of 0.10 m. The mean depth of river water in

the study area is approximately 3 m, varying in the

range 2-5 m, and the mean width is 64 m, varying

in the range 40-90 m. An underwater sill also

exists at the mouth of the estuary, approximately

200 m long and 0.2-0.8 m deep (Vouvalides,

1998). The river discharge varies in the range 0-

120 m3 s-1 (Hatzigiannakis et al., 2000) due to the

presence of the Kerkini dam in the pathway of the

river.

The wedge develops mostly in the summer, when

there is a great demand for irrigation water for

the nearby agricultural fields, as the freshwater

discharge from the dam is minimal. A critical

point in the estuary is at a distance of approxi-

mately 3 km from the mouth, where a pumping

station, which pumps water for irrigation, has

been installed. The pump is equipped with a salin-

ity sensor, which interrupts pumping when the

water quality is unsuitable for irrigation, i.e.,

when the salinity is greater than 15 psu.

Therefore, in order for the pump to operate, the

salt wedge should not extend beyond 3 km from

the mouth.

The field survey was conducted in winter of 2002

and in spring and summer of 2003. Three prelim-

inary sampling campaigns were conducted in win-

ter and spring for the study of seasonal variations

(Haralambidou et al., 2003a; 2004). The saline

wedge was absent at this period. The summer

campaigns were more intense because of the

presence of the saline wedge. Five sampling cam-

paigns were made in summer, namely one in

June, two in July and two in August. In most cases

in the summer the sampling lasted about 12h.

Temperature, conductivity, salinity, pH, dissolved

oxygen and velocity were measured along the cen-

terline of the estuary (Figure 3). Data were col-

lected every 0.25 m from surface to bottom and


Figure 2. The dependency of the behavior of the salt wedge on ratios A/B and A/R (Nakai and Arita, 2002).

Figure 1. Flow type classification according to Nakai and Arita (2002).


profiles of the above mentioned parameters were

obtained using conductivity, pH and dissolved

oxygen meters (WTW). The total number of sta-

tions along the estuary was variable, as the length

of the saline wedge was different based on river

discharge and tide.

NUMERICAL EXPERIMENTS AND RESULTSModel resultsEquations (1) to (6) were solved using the

method of finite-differences. The computer code

was written in FORTRAN 77 and was compiled

using PROSPERO FORTRAN. The grid was in

the vertical plane, with spacings set at Äx = 1000

m horizontally and Äz = 0.5 m vertically, and a

time step increment of Ät = 15 sec. A run in time

of two tidal periods was found adequate for the

tidal regime and the vertical profiles of velocity

and salinity to become established. The results of

the second period were used in the following

analysis. It was assumed that there was no salt flux

through the surface or bottom and that there was

no wind stress on the surface. The initial distribu-

tion of the salinity field was defined by the moni-

toring surveys with value at the river mouth of

approximately 35 psu and value at the head water

of 0 psu.

The parameters of L15 and L30 were introduced to

define the salinity intrusion length along the up-

estuary direction. The parameters L15 and L30

represent the salt intrusion length defined by the

bottom position of the 15 psu and 30 psu-isoha-

lines, respectiverly, measured from the river

mouth (Tuin et al., 1991). The model was used to

investigate the influence of the river discharge,

the tidal amplitude, the bottom Manning's rough-

ness coefficient and the topography of the estuary

on the salinity structure in the Strymon Estuary.

The conditions simulated by the model are sum-

marized in Table 1. Figure 5 illustrates a repre-

sentative salinity profile under the typical summer

flow condition of Q = 15 m3 s-1, with a spring tidal


Figure 3. Map of Strymon River system and its location in Greek territory.


range of DE = 45 cm and Manning's roughness

coefficient n = 0.050.

Effect of river discharge: To study the responses of

salinity distribution to varying river discharge, the

model was run for river discharge values Q=1 m3

s-1 to Q=30 m3 s-1, with tested values of 1, 5, 15, 20

and 30 m3 s-1, and with a spring tidal range of

DE=45 cm and a Manning's roughness coeffi-

cient n = 0.050 (Table 1). Figure 6a shows the

salinity intrusion lengths L15 and L30 as function

of river discharge, Q. It is apparent that the mag-

nitude of the salinity intrusion length depends

inversely on the magnitude of the river discharge,

as expected. It is also seen that the freshwater dis-

charge is a quite sensitive parameter in control-

ling salt wedge intrusion.

Effect of tidal amplitude: The model was also run

for different tide regimes. The neap tide, the

mean and the spring tides have been tested, with

amplitudes DE of 10 cm, 18 cm and 45 cm. The

river discharge was set to Q = 15 m3 s-1, and the

Manning's roughness coefficient to n = 0.050

(Table 1). The results are presented in Figure 6b.

It is apparent that there is a nearly exponential

increase of the salinity intrusion length with the

increase of tidal amplitude. For the given fresh-

water discharge, it is also seen that the neap tide

under these conditions the 30 psu isohaline does

not enter the river (L30 = 0) but the 15 psu isoha-

line progresses more than 4 km up-estuary.

Again, the tidal amplitude is a quite sensitive

parameter in controlling salt wedge intrusion.

Effect of Manning's roughness coefficient: The

effect of varying the Manning's roughness coeffi-

cient n was tried by making runs using n values

from 0.020 to 0.065, with tested values of 0.020,

0.035, 0.040, 0.050, 0.060 and 0.065, with constant

values of river discharge (Q = 15 m3 s-1) and tidal

amplitude (DE = 45 cm) (Table 1). The results

are shown in Figure 6c. It is apparent that a small

decrease of saline intrusion takes place as the

roughness coefficient increases.

Effect of the river mouth topography: The model

was run for different widths and underwater sill

heights at the mouth. The widths W have been

tested in the range from W=50 m to W=80 m,

with tested values of 50, 60, 70 and 80 m. In

another set of runs, the underwater sill heights at

the river mouth SH varied from SH=0.5 m to

SH=2 m, with tested values of 0.5, 1, 1.5 and 2 m

(i.e., the sill was raised). The parameters of river

discharge (Q=15 m3 s-1) and tidal amplitude

(DE=45cm) remained constant throughout these

tests (Table 1). A small increase in the length of

salinity intrusion was observed as the width

increased, which was more obvious on the 15 psu-


Table 1. Summary of conditions of numerical model experiments.

Case 1: Case 2:Case 3:

Case 4: Case 5:

Typical Effect of Effect ofEffect of

Effect of Effect ofParameter

Conditions Freshwater TidalManning's

Mouth Sill

Discharge AmplitudeRoughness

Width HeightCoefficient

FreshwaterVarying:

Discharge 151 - 30

15 15 15 15

(m3 s-1)

TidalVarying:

Amplitude 18 4510 - 45

45 45 45

(m)

Manning'sVarying:

Roughness 0.050 0.050 0.0500.020 - 0.065

0.050 0.050

Coefficient

Mouth80 80 80 80

Varying:80

Width (m) 50 - 80

Sill Height2.5 2.5 2.5 2.5 2.5

Varying:

(m) 0.5 - 2.0


isohaline than the 30 psu-isohaline. On the con-

trary, no significant change was observed as the

underwater sill height at the mouth rose (Figure

6d, 6e).

Air curtain resultsThe model was used to obtain the values of den-

sity of saltwater and freshwater for different val-

ues of river discharge Q, tidal amplitude DE,

Manning's roughness coefficient n, widths W and

sill height SH at the mouth of the estuary. The

conditions simulated by the model and tested

using the air curtain, are summarized in Table 2.

Flow types (i.e., I, II or III according to Figures 3

and 4) for the conditions mentioned above result-

ed from the calculation of the ratios A/R and A/B

(Eqs. 7). Figure 7 presents some of these results

based on the number of observations made. The

following are observed:

• As the river freshwater discharge increases the

flow type moves from Type I to II.

• For the same values of river discharge, an

increase in the air flow qa has as a conse-

quence the elimination of Type I condition

and the appearance of Type II. Type III

appears at minimum tested river discharge of

Q = 15 m3 s-1.

• There is no significant effect on flow type for

changes in the Manning's roughness coeffi-

cient, sill height at the river mouth and width

of the channel. However, there is an impact on

the flow type when the discharge of the air

increases. In particular for qa = 0.01 m3 s-1 per

unit width all conditions of Manning's rough-

ness coefficient tested are of Type I and for qa

= 0.05 m3 s-1 per unit width most conditions

are of Type III. Exactly the same happens in

the cases of varying sill height and width of the

channel.

• In relation to tidal amplitude, the neap tide

condition appears to be out of the range of the

diagram, whereas the spring tide condition is

of Type I and the mean tide condition is of

Type II. As qa rises mean tide condition

remains of Type II and spring tide condition

moves from Type I to Type III.

PROPOSALS FOR THE MANAGEMENT OF THESALT WEDGEFrom the sensitivity analysis of the model it is

obvious that:

1. River freshwater discharge is the most sensi-

tive parameter. Thus, if no other measures are

taken to solve the problem, a minimum con-

stant flow of 30 m3 s-1 should be negotiated by

authorities and be released from Kerkini dam

at all times. This flow would keep the 15 psu

isohaline downstream of the pumping station

even during spring tides.

2. The tidal amplitude is also a very sensitive

parameter, affecting salt wedge intrusion.

3. Manning's roughness coefficient has only a

minor effect on the propagation of the salt

wedge. Thus, roughening the riverbed by

installing artificial concrete blocks or dumping

rock and crushed stone would be a rather

expensive and ineffective solution. This could,

however, be done in conjunction with another

measure.

4. Reducing the width of the river mouth has

some minor impact on the salt wedge propa-

gation. It could be done relatively inexpensive-

ly, but since it cannot eliminate totally the

problem, it should be done in conjunction with

another measure.

5. Raising also the sill height at the mouth did

not show to have a significant impact on the

propagation of the salt wedge. Furthermore,

this measure would create problems with

small fishing boats passing through the mouth.

Thus, this should not be an option, unless, as

mentioned above, the tide is totally blocked,

for example, with an inflatable dam.

From the application of the air curtain method it

is apparent that:

1. When the river discharge is greater that Q = 30

m3 s-1, a discharge of air qa of the order of 0.01

m3 s-1 per unit width is sufficient to produce

flow of Type II. On the contrary, for river dis-

charge less that Q = 30 m3 s-1 an increase in qa

improves the situation from Type I to Type II.

2. In relation to varying Manning's roughness

coefficient and characteristics of the topogra-

phy of the estuary, an increase in qa does not

alter the type of the flow and is considered to

be meaningless.

3. Under spring tide conditions high values of qa

are more effective, as they alter the condition

from Type I to Type III. However, under

mean tide condition there is no need to alter

the qa value.



CONCLUSIONSA two-dimensional, laterally-integrated, explicit

finite-difference numerical model was developed

to describe salt wedge dynamics in Strymon River

estuary. This model was applied successfully and

proved to be an effective tool in testing alterna-

tive management scenarios for controlling salt

wedge intrusion in this estuary. The effect of river

freshwater discharge, tidal amplitude, bottom

Manning's roughness coefficient, river mouth

width and underwater sill height at the river

mouth was also studied. Finally, the model pro-

vided the required minimum air flow value for air

curtain technology application to stop salt wedge

upstream progression under given river flow and

tide amplitude conditions.


Table 2. Summary of conditions tested using the air curtain method for density of fresh water ñf = 1.005396 g

cm-3, total water depth d = 3 m and testing discharges of air qa = 0.01 and 0.05 m3 s-1 per unit width.

Parameter qf(m3/s per unit width) ñ

s(g/cm3)

River Discharge Q (m3/s)

15 0.229 1.0224

20 0.305 1.0179

30 0.457 1.0098

40 0.610 1.0084

50 0.762 1.0080

Tidal Amplitude DE (cm)

10 0.229 1.0055

18 0.229 1.0101

45 0.229 1.0224

Manning's Roughness Coefficient n

0.020 0.229 1.0290

0.030 0.229 1.0282

0.040 0.229 1.0258

0.050 0.229 1.0224

0.060 0.229 1.0190

0.065 0.229 1.0174

Width of Estuary Mouth W (m)

50 0.229 1.0199

60 0.229 1.0218

70 0.229 1.0225

80 0.229 1.0224

Underwater Sill Height SH (m)

0.5 0.229 1.0241

1.0 0.229 1.0236

1.5 0.229 1.0229

2.0 0.229 1.0224

qf = discharge of freshwater flow per unit width; ñs = density of saltwater



Figure 5. Longitudinal salinity distribution computed by the model under river discharge of Q = 15 m3/s at

spring tide range DE = 45 cm and with Manning's roughness coefficient n = 0.050. The lines are iso-

halines and the labels represent salinities in psu. The x-axis represents number of Äx's (Äx = 1000 m)

and the z-axis number of Äz's (Äz = 0.5m).

Figure 4. Longitudinal profiles of salinity upstream of the mouth of Strymon River measured on 31-08-2003 at:

(a) 11:00 am; (b) 13:00 pm; (c) 15:30 pm; and (d) 18:00 pm. The x-axis is in m and the z-axis is in cm.

Isohaline labels are in psu.



Figure 6. The relation of salinity intrusion length, LS to: (a) the river discharge Q; (b) the tidal amplitude DE;

(c) the Manning's roughness coefficient n; (d) the width of the estuary mouth W; and (e) the under-

water sill height at the estuary mouth SH.



Figure 7. Diagrams of various conditions for two flowrates of air.

14-Haralambidou.qxd 9/11/2005 10:04 7


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